The present invention relates to a method for determining the remaining useful life, or life expended, of turbine components and, more particularly, to determining the remaining useful life of turbine components which generally operate at relatively high temperature and are thus employed in an environment wherein creep of materials constituting the turbine components becomes a major factor in determining remaining useful life of the components.
Current estimates of electrical energy production over the next twenty years show a critical dependence on steam power plants and associated turbines with over thirty years of service. Traditionally, utility plants and associated turbines of this age would be retired and replaced with new units. However, in the current environment of depressed load demand and high cost of new construction, utilities are increasingly relying on life extension programs for these older plants in order to meet their expected future power delivery requirements. Practical and business considerations require that these life extension programs be implemented while maintaining traditional levels of availability, performance and reliability. Achievement of an optimum balance between investment capital and required return, necessitates evaluation of existing condition and probable future performance of critical turbine components, as well as realistic assessment of risks associated with various life extension options.
Evaluating present condition and determining probable future performance of turbine components, especially for those components that operate in the creep regime of materials constituting the components, present a challenge because of the complexity of turbine components, the variety of in service operating conditions experienced by the components and the inherent limitations of prevailing remaining useful life, or life expended, estimation methods. Components which operate at high temperatures (i.e. greater than about 900.degree. F.), where a combination of creep and thermal fatigue of the material constituting the components is of prime concern, demand special consideration in order to achieve an acceptable remaining useful life estimation.
A variety of techniques are currently used for assessing remaining useful life of power plant components. These techniques can be generalized into two broad categories: destructive and/or non-destructive testing of the actual component, and analytical estimation by use of material behavior and component operating history.
Prior techniques using destructive or non-destructive examinations have been found to have limitations when applied to major turbine components. It is often difficult to obtain material for destructive testing from critical areas of these components and to gain suitable access to many critical regions of the turbine for non-destructive testing. In addition, while some prior non-destructive techniques may provide estimates of remaining useful life of a component that is subject to pure creep loading, normal operation of many turbine components subject them to combined creep and fatigue damage, the fatigue being quite significant in determining life expended, or used up, in the component. Creep, which is a function of the time interval during which stress is applied, is inelastic, or unrecoverable (i.e. unable to return to its original shape and state), deformation of a material. Fatigue, which is not time but stress cycle dependent, is a form of plastic strain that may ultimately cause a component to rupture. Prior techniques have not been able to adequately evaluate the magnitude of damage experienced from a combination of creep and fatigue. Another technique which has been used, but which has not provided adequate results, employs creep void density as an indication of expended creep life. Thus, these prior techniques do not generally yield results having the desired degree of accuracy on which to base recommendations so as to aid the decision making process for evaluating and comparing potential turbine extention strategies.
Analytical estimation of expended life (which then may be subtracted from estimated total life to yield remaining useful life) generally utilizes sophisticated material behavior representations, damage assessment rules, and actual (or idealized) past and future operating conditions. The accuracy of any particular analytical approach depends on the ability of the method to deal with uncertainties associated with actual operating components.
For example, in U.S. Pat. No. 4,046,002--Murphy et al, assigned to the present assignee, the method for determining rotor life expended is based on using low cycle fatigue damage, which is stress cycle dependent, and not creep rupture damage, which is time dependent. The stress range for each cycle is compared with a calculated stress range curve for the turbomachine part to determine the amount of life of the turbomachine part expended as a result of the cycle. The time interval between local stress peaks used to determine a stress cycle is not considered.
In U.S. Pat. No. 3,950,985--Buchwald et al, a method based on Miner's hypothesis of linear accumulation of damage is used. Miner's hypothesis may be expressed by equation (a): ##EQU1## wherein t(.sigma.,.theta.) is the time to rupture for a stress .sigma. and temperature .theta.. That is, Miner's hypothesis states that failure occurs when the integral on the left of equation (a) equals one. According to U.S. Pat. No. 3,950,985, the value of t(.sigma.,.theta.) of equation (a) is determined from the graph of FIG. 1. Thus, this is a stress based method which does not consider the amount of creep strain accumulated.
Accordingly, it is an object of the present invention to provide a method for accurately determining remaining useful life, or life expended, of turbine components.
Another object of the present invention is to provide a method for accurately determining remaining useful life, or life expended, of turbine components while including the effects of temperature stress, creep strain accumulation, and rate of creep strain accumulation.